Interpretive Summary: Listeria monocytogenes is a human pathogen that has been associated with fully cooked poultry products. This organism can generally be found on a low percentage of broiler carcasses at slaughter while prevalence tends to increase during processing. Raw poultry meat is the most important documented source of L. monocytogenes entering a commercial poultry cooking plant. Once a cooking plant has been exposed, L. monocytogenes can colonize the plant and become a long term resident in floor drains. We recently collected data that demonstrates the potential for Listeria to become airborne during standard plant wash down when the drains are cleaned. Aerosolized Listeria is a significant hazard and one source of the organism capable of contaminating raw or even fully cooked poultry meat. Interventions are needed to control L. monocytogenes at the point of transfer of raw meat to a cooking plant as well as in the floor drains themselves. We conducted work demonstrating that germicidal ultra-violet light can effectively lower the number of L. monocytogenes on raw poultry meat prior to transport to a cooking plant. We also showed that floor drain sanitizers, especially peroxide based sanitizers, can effectively lower the numbers of L. monocytogenes both in floor drain liquid and attached to the inner surfaces of the drain pipe as a biofilm. ARS research has identified L. monocytogenes as a source of contamination through transfer to cooking plants, colonization of floor drains and subsequent airborne dissemination and contamination. Additionally, we have published reports demonstrating possible intervention strategies and continue to conduct research to develop new, and optimize existing, interventions.

Technical Abstract:
Listeria monocytogenes is recognized as an important food borne pathogen. Although relatively uncommon, food borne listeriosis has a high mortality rate in susceptible individuals. This pathogen has been found in poultry further processing plants and can contaminate fully cooked ready-to-eat poultry. A 2002 outbreak of listeriosis in the U.S. which resulted in 8 deaths and 3 miscarriages or still births was traced to fully cooked poultry meat products (1).
Unlike Salmonella or Campylobacter, L. monocytogenes does not enter the broiler slaughter plant in large numbers with the live bird (2). Prevalence of L. monocytogenes on broiler carcasses can increase during passage through the slaughter and processing plant (3, 4). However, after the implementation of the U.S. Department of Agriculture’s Food Safety Inspection Service pathogen reduction ruling in poultry slaughter establishments, we were unable to consistently detect a high prevalence of L. monocytogenes on chilled carcasses (5). Therefore, we focused our energy on further processing as the most important area of concern for L. monocytogenes.
We examined 40 environmental sites within a poultry further processing plant and detected L. monocytogenes in more than half of them. Examination of isolates by subtyping showed that multiple types of L. monocytogenes were present in the environment of this specific plant (6). Next, we repeatedly examined two different chicken further processing plants, one over the course of one year (7) and the second, a newly constructed facility, over the course of two years (8). L. monocytogenes was isolated from multiple environmental sites in both facilities. We tested potential sources of the organism to the cooking plant and found that we could document the entry of L. monocytogenes on incoming raw product (cut up chicken parts from separate slaughter plants) (8). Upon molecular subtyping we discovered that in some cases subtypes recovered from raw product were repeatedly isolated from floor drains even after clean up of the plant (8). This demonstrates that further processing plants can become colonized by persistent strains of L. monocytogenes which enter the plant on or with raw chicken meat.
L. monocytogenes in floor drains is a concern because of the potential to escape and contaminate product or product contact surfaces. We tested this possibility by creating Listeria biofilms in model floor drains, exposing them to a two second spray from a water hose and measuring airborne Listeria. We found that a short hose spray, even if done accidently, can result in the airborne dissemination of Listeria to points at least 4 m distant from the drain and 2.4 m above the floor (9). This also demonstrates the potential for contamination of raw chicken meat due to hose spray into a contaminated floor drain.
By gaining an understanding of processing plant microbial ecology and potential for product contamination we were able to focus our intervention efforts for best effect. We tested chemical and physical means to decontaminate floor drains. By use of chemical sanitizers and ultrasonic disruption of biofilm architecture, we were able to significantly lower (P<0.05) the number of both planktonic and attached L. monocytogenes cells in model floor drains (10). We tested the antimicrobial efficacy of silver ions immobilized in a plastic work surface and found that under the conditions tested, L. monocytogenes was equally able to survive and form biofilms with or without the presence of ionic silver (11).
In an attempt to develop a means to break the cycle of contamination transferred from a slaughter plant to a cooking plant with raw product, we tested the use of germicidal ultra-violet light to kill L. monocytogenes on raw chicken meat. We found that a 5 min treatment with 1000 µW/cm2 was effective to kill about 99% of L. monocytogenes without altering the organism’s antibiotic resistance profile (12). Currently, we are working on optimization of several interventions to help control L. monocytogenes in poultry processing and further processing.